Thermoplastic polymers, for example, vinyl aromatic polymers such as polystyrene, can be made expandable by incorporating an expandable agent in the polymeric matrix. Typical expanding agents for vinyl aromatic polymers include at least one liquid hydrocarbon containing from 3 to 7 carbon atoms, a halogenated hydrocarbon, carbon dioxide or water. The quantity of expanding agent ranges from 2 to 15% by weight.
Expandable polymers are produced, in general, as beads or granules which, under the action of heat, supplied, for example, by steam, are first expanded until a desired density is reached and, after a certain aging period, are sintered in closed molds to produce blocks or the desired final products.
Expandable beads can be obtained by means of a batch polymerization process in suspension. This process, which is well-known as described in U.S. Pat. No. 2,673,194 and U.S. Pat. No. 4,500,692, which comprise the following steps:                dispersing the liquid vinyl aromatic monomer in an aqueous medium using suspension agents;        adding polymerization catalysts and polymerizing the vinyl aromatic monomer in a stirred, heated reactor;        adding the expansion agent at a certain degree of conversion of the monomer;        completing the polymerization;        discharging, washing, drying and sieving the resulting beads. The diameter of the beads generally ranges from 0.1 to 3 mm.        
That suspension process allows a good quality product to be obtained but has various drawbacks, such as:                a very wide distribution of the bead diameter, followed by the necessity of separating the different fractions by screening and discarding the tails;        significant limits in the production of special products, such as colored beads and/or beads which contain heterogeneous fillers or additives, for example nucleating agents and/or flame-retardant agents, as it is difficult to incorporate them into the beads or they can inhibit the polymerization. See, for example, U.S. Pat. No. 4,360,611, or WO 98/51734 and WO 00/29471;        further limits for the use of expanding agents, which must be dispersible and/or soluble in the polymeric matrix or in the use of monomers, which must be capable of polymerizing in aqueous solution;        difficulty in reducing the residual monomer, for example styrene, in the polymer, below 1,000 ppm, and in reducing the total internal water content;        environmental problems due to the considerable water consumption which cannot be simply discharged into the sewers, unless subjected to purification treatment, and to the impossibility of recycling the expanded polymer after use.        
These and other drawbacks can be overcome by means of a mass-continuous process in which a polymeric composition in the molten phase is mixed with solid additives and expanding agents. These processes are described, for example, in GB-A-1,062,307 and EP-A-668,139. A process is described in these patents for the production of granules of expandable polystyrene (EPS) according to which the molten polymer and the expanding agent are mixed by means of static mixing elements. After cooling, the mix is granulated through a die.
The product thus obtained generally has the drawback of having a cell structure, after expansion, wherein the cells are irregular and normally too large. The number of cells and the structure of the foam obtained during the expansion play a very important role in obtaining ideal thermal insulation properties and a good surface on the foams. For this reason, the use of nucleating agents is often necessary. EP-A-126,459 describes a process for overcoming these problems by means of a thermal treatment of granules carried out under pressure and at a temperature higher than the glass transition temperature of the expandable polymer.
Furthermore, the expandable resin which leaves the die is difficult to cut due to its natural tendency to expand. In US 2006/167123 a process is described which exploits this characteristic to obtain low density expanded granules by means of the direct cutting of the expandable molten polymer exiting the die.
In other processes, described for example in U.S. Pat. No. 5,108,673 and U.S. Pat. No. 5,573,790, the expandable polymer is extruded into continuous filaments, immediately cooled with cold water at the outlet of the die and granulated in a subsequent step. It is not possible, however, to obtain substantially spherical beads, as the cutting of the filament produces cylindrical pellets with diameters normally larger than 1 mm and the equipment must be kept under pressure to avoid the pre-expansion of the pellets.
The incorporation of organic charges can be problematic. WO 00/43442 states that athermanous materials have a strong nucleating effect, and consequently, to prevent pre-expansion, it is necessary to operate with an underwater granulation system and under pressure. That process comprises the use of a particular die, wherein the expandable polymer is extruded through a series of small holes. Experts in the field know that this process is extremely binding as the temperature of the die surface is close to that of water and this leads the polymeric flow close to reach its solidification temperature. Due to the swelling during extrusion and to the necessity of producing extruded granulates having very reduced dimensions, very small holes are required, with diameters ranging from 0.3 to 1.2 mm. Consequently, extrusion through these holes requires very high shear deformation rates. This implies that, with this technology, extruded particles having dimensions lower than 1 mm cannot be obtained.
US 2005/156344 describes the influence of the geometry of the die holes (such as the diameter of the hole, the length/diameter ratio, the angles of the inlet and outlet cones), of the temperature of the molten product and of plasticizers on the swelling of the polymer at the outlet of the die holes and therefore on the diameter of the final bead. It describes that the expandable resin can contain various additives, such as nucleating agents, plasticizers, flame-retardants, organic or inorganic dyes and pigments, soluble or insoluble, such as carbon black, graphite or aluminium, up to 30%.
One of the problems known in the literature relating to the production of beads of mass-continuous expandable vinyl aromatic polymers is the necessity of cooling the polymer containing the expanding agent, before the extrusion of the same through the die. This cooling can be carried out, for example, by means of cooling elements inside the extrusion devices, by means of heat exchangers or through static mixing equipment with embedded cooling devices.
On the basis of this known art, the cooling of the polymer before granulation is essential to prevent the expansion of the same in the granulator and to obtain products having a regular shape.
EP 668,139, describes a process for the preparation of expandable plastic granulates having a diameter at least lower than 1 mm. According to that process, the temperature of the expandable molten polymer must be reduced to a few degrees above the solidification temperature of the polymer itself, to obtain these granulates.
It is well-known that the viscosity of thermoplastic polymers increases considerably when the temperature of the molten polymer approaches the solidification point. It is apparent that, when high viscosity is combined with high shear deformation rate, corresponding to the passage of the polymer through the die holes, a considerable pressure drop occurs when the polymeric flow passes through the die. This requires the use of particular dies, suited to mechanically sustain these pressure gradients.
Furthermore, the rapid cooling of the polymer, due to the submersion of the die surface under water (“underwater” configuration) makes the plugging of the die holes and flow instability extremely probable.
US 2005/140039 and US 2005/123638 describe a process (the former) and equipment for hot granulation (the latter) for the preparation of granules of expandable thermoplastic polymers. According to those patent applications, the obstructions and irregularity of the flow mentioned above can be prevented by means of a new granulation device wherein the molten polymer is extruded and immediately cooled by means of a chopped water spray and not underwater. Consequently, the expansion of the granulated product is avoided and, at the same time, plugging is limited. No information is provided on the procedure or on the production process of expandable beads containing a high quantity of additives.
EP 1,702,738 describes a process and equipment for the preparation, in continuous, of granules of expandable polymers. According to that application, plugging of the die holes can be avoided by constructing a specialized electronic control system which controls the process conditions and maintains the pressure and temperature of the molten polymer within a pre-established range. Systems based on an active electronic control of the stability, however, are not generally considered as being intrinsically reliable. Therefore they do not allow a high reliability of the plant and consequently their use is generally not advisable for large-scale production.
WO 2006/88392 describes a process which enables the continuous production of polystyrene and high impact polystyrene in the form of expandable granules, within a wide range of molecular weights. The innovation is the mixing system between the molten polystyrene and the expanding agent. The expandable beads are prepared by extrusion and cooling of the molten polymer and subsequent granulation. No particular data are supplied with respect to the quality of the resulting expandable granule and more specifically of the expandable granule with a high content of inorganic additives.
EP 1,693,413 describes a process for continuous production of expandable polystyrene particles containing inorganic silicates and zeolites to improve the insulating properties of the corresponding foams. The polymer is fed to an extruder and mixed with an expanding agent and with additives. It is then extruded, cooled and cut into particles. No indication is provided with respect to the granulator configuration, process conditions, the dimension and the shape of the particles obtained.
The expandable vinyl aromatic polymer particles obtained in a mass-continuous process can be badly affected by the orientation and mechanical stress due to the extremely rapid cooling to which the particles are exposed when granulated. When these granules are expanded and molded, this stress causes heterogeneity in the cellular structure which, in turn, negatively influences the mechanical and aesthetical properties of the manufactured final products. US 2005/140039 claims that, to solve these drawbacks, this stress can be released by annealing the granules at a temperature close to the glass transition temperature (Tg) or by means of nucleating additives. In any case, the effect of the inorganic additives on the nucleation has not been explored.